Analysis of the cause of water ingress and moisture in distribution transformers
1. Judgment
Take a small amount of oil sample from the sampling valve at the bottom of the distribution transformer. If the oil sample appears cloudy and whitish, a simplified analysis reveals that the insulation strength test value is close to or below the limit value of 25kV. This can be roughly determined that the distribution transformer has been affected by water and moisture. Having worked in oil testing for many years, I have found that water and moisture ingress account for one-third of all unqualified oil samples.
2. Harm
Moisture ingress into distribution transformers poses a significant threat to the electrical and physicochemical properties of the insulating media. The presence of moisture weakens the oil's insulation strength, accelerates the corrosion of metals such as copper and iron by organic acids, and catalyzes oil oxidation, causing the oil to age 2-4 times faster than when dry. It can even permanently damage the paper insulation. If not addressed promptly, this can lead to grounding breakdown or inter-turn short circuits in the coils, resulting in equipment damage.
3. Reasons
During normal operation, the temperature of a distribution transformer changes throughout the day with variations in ambient temperature and load. When the temperature rises, the insulating oil inside the transformer expands, displacing excess oil from the transformer body into the oil conservator. This increases the oil level in the conservator, expelling excess air and maintaining pressure balance. The process reverses as the transformer temperature drops. This "breathing" process occurs daily and is accomplished by a breather screw on the upper part of the oil conservator. During the day, when temperatures are high, the transformer exhales air; at night, when temperatures are low, it draws in air. Since the humidity is higher at night than during the day, this exhalation of low-humidity air and intake of high-humidity air creates a humidity difference. Over time, the accumulated moisture condenses on the inner wall of the oil conservator, dripping into the oil and settling at the bottom (this process is more pronounced during rainy and humid seasons). Subsequently, the accumulated moisture is carried into the transformer body by the breathing oil flow.
4. Prevention
(1) Before adding oil to the transformer, the oil should be tested to prevent moisture from entering the oil during storage due to improper storage.
(2) Before refueling, drain a small amount of fuel from the fuel tank. Align the fuel dispenser with the fuel filler neck and slowly add fuel. The fuel level should be between 1/4 and 3/4 of the fuel gauge.
(3) When refueling, it should be done on a dry, sunny day at noon, and the oil should be stored in a clean and dry container.
(4) Prevent oil shortage in distribution transformers. When the transformer oil level drops below the transformer casing, the contact area between the oil and air increases, and the oil easily absorbs moisture from the air.
(5) For large-capacity transformers, a breather should be used to prevent moisture accumulation by using a desiccant.
(6) For small-capacity transformers, breathers are generally not required. However, the height of the oil pipe connecting the transformer body and the oil tank should be appropriately increased at the oil pipe opening inside the oil tank to prevent moisture from entering the transformer body.
